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Lab-Made Antennas Are Sun ‘Sponges’

Photonics.comAug 2013
ST. LOUIS, Aug. 27, 2013 — The evolutionary chemical machinery of nature — and a dash of human ingenuity — have created synthetic light-harvesting antennas that convert sunlight into unprecedented amounts of usable energy.

Organized by the Photosynthetic Antenna Research Center (PARC) at Washington University in St. Louis, the project is a platform — called a test bed — for rapid prototyping of light-harvesting antennas, which are normally found in plants and photosynthesizing bacteria. They consist of carefully designed ring of proteins with attached pigment molecules that are in ideal positions to capture the sun’s energy, including wavelengths that plants naturally ignore.

The team tapped the expertise of many PARC-affiliated scientists, including WUSTL's Dewey Holten and Christine Kirmaier; Paul Loach and Pamela Parkes-Loach of Northwestern University; Jonathan Lindsey of North Carolina State University; David Bocian of the University of California, Riverside; and Neil Hunter of the University of Sheffield in England.

Graduate student Michelle Harris and research scientist Darek Niedzwiedzki in PARC’s Ultrafast Laser Facility. The laser setup allows them to measure energy transfer steps among pigments in light-harvesting antennas that take place in a trillionth of a second. Courtesy of Angeles/WUSTL.
One of the two prototype antennas built on the test bed incorporates synthetic dyes called Oregon Green and Rhodamine Red, while the other combines Oregon Green and a synthetic version of the bacterial pigment bacteriochlorophyll, which absorbs light in the near-infrared. Both designs soak up more of the sun’s spectrum than the native antennas in purple bacteria that provided the inspiration and some of the components for the test bed.

While plants miss the middle of the visible spectrum, they also miss wavelengths that are too long to see, such as the near-IR photons absorbed by photosynthetic bacteria. Accessory pigments such as carotenoids fill some gaps, but large swaths of the solar spectrum pass through untouched.

“Since plant pigments actually reject a lot of the light that falls on them, potentially there’s a lot of light you could gather that plants don’t bother with,” Hunter said.

Antennas are made up of modules: a two-peptide dyad (side view shown) with the pair of bacteriochlorophyll molecules (purple) in the middle. The bacteriochlorophylls absorb light and trap the energy transferred from other pigments. The additional pigments are attached at carefully chosen sites (red, orange and yellow) on the beta peptide (the green helix), or at the top end of the alpha peptide (blue helix). Courtesy of COGDELL/Holten/PARC.
To design and synthesize pigments that can absorb at wavelengths that will fill some of the holes in the absorption of natural systems, the team relies on Jonathan Lindsey.

“It can’t be done from first principles, but we have a large database of known absorbers, and so drawing on that and reasoning by analogy, we can design a large variety of pigments,” Lindsey said. “The effectiveness of the design depends not only on having extra pigments but also pigments able to talk to one another, so that energy that lands on any one of them is able to hop onto the next pigment and then to the next one after that. They have to work together.”

“The energy cascades down like a waterfall,” Hunter said. “So you pour the energy at the top of the waterfall, and it hits one pigment and jumps to the next and the next and finally to the pigment at the bottom, which in terms of energy is the pigment that is reddest in color.”

The scientists also aimed to avoid the laborious synthesis typically required to make designer light-harvesting antennas. Thanks to the chemical affinities of purple bacteria antennas, the proteins self-assemble into dyads when added together in detergent. Adjusting the detergent concentration and temperature induces the dyads to form rings, which in native antennas contain up to 16 alpha/beta dyads and thus as many as 32 bacteriochlorophylls.

In the test bed, the scientists use peptides that have been slightly modified from the native amino acid sequence for attachment of the extra pigments to increase solar spectral coverage. The attachment sites were chosen to avoid disrupting the self-assembly of the components into dyads and dyads into rings.

“This is an example of what the field would refer to as semi-synthesis,” Lindsey said. “We take naturally occurring materials and combine them with synthetic ones to make something that doesn’t exist in nature. By taking lots of material from nature, we can make molecules that are architecturally more complex than those we can make from scratch.”

The dyad modules assemble into rings (side view, left; top view, right) that pack many pigments together for light harvesting. Because of the additional pigments attached to the dyads, the antennas absorb more of the sun’s light than natural systems. Courtesy of HUNTER/Holten/PARC.
Once assembled, the antennas are sent to the Holten-Kirmaier lab, where a variety of spectroscopic methods are used to excite each pigment molecule. The energy transfer is followed from one pigment to the next, all the way down to the bacteriochlorophyll target. With the correct pigments in the right locations, the transfer is extremely efficient with little energy lost on the way.

“I’ve been working in photosynthesis for 50 years, and I can’t think of many other times when there were so many good people with so many different talents coming together to try to solve problems,” said Parkes-Loach. “It’s fun to be part of it and to see what comes out of the collaboration.”

One day, a two-part system, consisting of an antenna and a second unit called a reaction center, may serve as miniature power outlets into which photochemical modules could be plugged. The sun’s energy could then be used directly to split water, generate electricity, or build molecular-scale devices.